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<HTML><HEAD><TITLE>Recipe 11.10. Reading and Writing Hash Records to Text Files (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:40:49Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch11_01.htm"TITLE="11. References and Records"><LINKREL="prev"HREF="ch11_10.htm"TITLE="11.9. Constructing Records"><LINKREL="next"HREF="ch11_12.htm"TITLE="11.11. Printing Data Structures"></HEAD><BODYBGCOLOR="#FFFFFF"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_10.htm"TITLE="11.9. Constructing Records"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 11.9. Constructing Records"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch11_01.htm"TITLE="11. References and Records"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_12.htm"TITLE="11.11. Printing Data Structures"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 11.11. Printing Data Structures"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch11-chap11_reading_0">11.10. Reading and Writing Hash Records to Text Files</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1090">Problem<ACLASS="indexterm"NAME="ch11-idx-1000004849-0"></A><ACLASS="indexterm"NAME="ch11-idx-1000004849-1"></A><ACLASS="indexterm"NAME="ch11-idx-1000004849-2"></A></A></H3><PCLASS="para">You want to read or write hash records to text files.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1096">Solution</A></H3><PCLASS="para">Use a simple file format with one field per line:</P><PRECLASS="programlisting">FieldName: Value</PRE><PCLASS="para">and separate records with blank lines.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1106">Discussion</A></H3><PCLASS="para">If you have an array of records that you'd like to store and retrieve from a text file, you can use a simple format based on mail headers. The format's simplicity requires that the keys have neither colons nor newlines, and the values not have newlines.</P><PCLASS="para">This code writes them out:</P><PRECLASS="programlisting">foreach $record (@Array_of_Records) { for $key (sort keys %$record) { print "$key: $record->{$key}\n"; } print "\n";}</PRE><PCLASS="para">Reading them in is easy, too.</P><PRECLASS="programlisting">$/ = ""; # paragraph read modewhile (<>) { my @fields = split /^([^:]+):\s*/m; shift @fields; # for leading null field push(@Array_of_Records, { map /(.*)/, @fields });} </PRE><PCLASS="para">The <CODECLASS="literal">split</CODE> acts upon <CODECLASS="literal">$_ </CODE>, its default second argument, which contains a full paragraph. The pattern looks for start of line (not just start of record, thanks to the <CODECLASS="literal">/m</CODE>) followed by one or more non-colons, followed by a colon and optional white space. When <CODECLASS="literal">split</CODE>'s pattern contains parentheses, these are returned along with the values. The return values placed in <CODECLASS="literal">@fields</CODE> are in key-value order, with a leading null field we shift off. The braces in the call to <CODECLASS="literal">push</CODE> produces a reference to a new anonymous hash, which we copy <CODECLASS="literal">@fields</CODE> into. Since that array was stored in order of the needed key-value pairing, this makes for well-ordered hash contents.</P><PCLASS="para">All you're doing is reading and writing a plain text file, so you can use related recipes for additional components. You could use <ACLASS="xref"HREF="ch07_12.htm"TITLE="Locking a File">Recipe 7.11</A> to ensure that you have clean, concurrent access; <ACLASS="xref"HREF="ch01_14.htm"TITLE="Escaping Characters">Recipe 1.13</A> to store colons and newlines in keys and values; and <ACLASS="xref"HREF="ch11_04.htm"TITLE="Taking References to Hashes">Recipe 11.3</A> store more complex structures.</P><PCLASS="para">If you are willing to sacrifice the elegance of a plain textfile for a quick, random-access database of records, use a DBM file, as described in <ACLASS="xref"HREF="ch11_15.htm"TITLE="Transparently Persistent Data Structures">Recipe 11.14</A>.<ACLASS="indexterm"NAME="ch11-idx-1000004851-0"></A><ACLASS="indexterm"NAME="ch11-idx-1000004851-1"></A><ACLASS="indexterm"NAME="ch11-idx-1000004851-2"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1146">See Also</A></H3><PCLASS="para"><CODECLASS="literal">The</CODE> <CODECLASS="literal">split</CODE> function in <ICLASS="filename">perlfunc </I>(1) and <ACLASS="olink"HREF="../prog/ch03_01.htm">Chapter 3</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; <ACLASS="xref"HREF="ch11_10.htm"TITLE="Constructing Records">Recipe 11.9</A>; <ACLASS="xref"HREF="ch11_14.htm"TITLE="Storing Data Structures to Disk">Recipe 11.13</A>; <ACLASS="xref"HREF="ch11_15.htm"TITLE="Transparently Persistent Data Structures">Recipe 11.14</A></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_10.htm"TITLE="11.9. 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